A power module which includes a power semiconductor device (for example, IGBT (Insulated Gate Bipolar Transistor), MOSFET (Metal Oxide Semiconductor Field Effect Transistor), bipolar transistor, diode, or the like) mounted on a circuit board and which is packaged with a sealant resin is used for example in a motor drive apparatus or the like.
A package structure called case structure is mainly used for the power module. This case-type structure is a structure in which a power semiconductor device is mounted on a heat-dissipation metal base plate with an insulating substrate interposed therebetween and a case is bonded to the heat-dissipation metal base plate.
The power semiconductor device mounted in the module is connected to a main electrode. For connecting the power semiconductor device to the main electrode, a wire is used. As the wire, a wire made of aluminum alloy and having a diameter of 0.1 to 0.5 mm is generally used.
In the case where wires are ultrasonically welded, it is necessary to set the distance between wires adjacent to each other so as not to cause a head of an ultrasonic welding tool to interfere with a wire having already been mounted. In order to obtain a power module for higher current, it is necessary to increase the number of wires joined to the power semiconductor device. However, due to limitation of the size of the power semiconductor device, the number of wires which can be mounted is limited, and there has been a problem that a power module for higher current is difficult to obtain.
With the aim of solving this problem, direct lead bonding has been proposed and put to practical use, as a technique replacing the wire bonding. The direct lead bonding joins a plate-shaped main terminal (lead) to the power semiconductor device with solder. The direct lead bonding has a feature that it can be adapted to higher current and can reduce the interconnect resistance and the interconnect inductance, as compared with the wire bonding (PTD 1 for example).
However, in the case of the direct lead bonding, stress caused by a difference in linear expansion coefficient between the power semiconductor device and the plate-shaped main terminal is exerted on the solder portion which is a joint material, to thereby cause cracks in the solder, resulting in a problem that the reliability is deteriorated. In contrast to this, a method has been disclosed according to which many metal bumps are formed on a surface of a plate-shaped main terminal and this conductor plate is joined to a surface electrode of a power semiconductor device via the metal bumps by ultrasonic welding (PTD 2 for example). In the case of this technique, the many metal bumps serve as a buffer layer to alleviate the stress exerted on the joint.